Electric discharge lamp with space charge relieving means



F. W. HOEH April 2, 1968 ELECTRIC DISCHARGE LAMP WITH SPACE CHARGE RELIEVING MEANS.

Filed Dec. '7, 1964 FIG].

INVENTOR Frederick W Hoeh BY ZIP- AGENT United States Patent 3,376,457 ELECTRIC DISCHARGE LAMP WITH SPACE CHARGE RELIEVlNG MEANS Frederick W. Hoeh, Livingston, N.J., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 7, H64, Ser. No. 416,263 6 Claims. (Cl. 313-205) ABSTRACT OF THE DISCLOSURE The anode voltage drop in a fluorescent lamp is reduced by mounting a pair of electrically-floating forarninous members, such as a pair of planar wire-grids, in spaced tandem relationship a predetermined distance in front of the anode. The distance between the foraminous members is such that the space therebetween constitutes a region along the discharge path that relieves the negative space charge near the anode.

This invention relates to electric discharge lamps and has particular reference to a fluorescent lamp having grid structures that reduce the anode voltage drop and thus improve the efiiciency of the lamp.

One of the problems encountered in the design of lowpressure gaseous discharge devices, such as fluorescent lamps, is the power loss caused by the voltage drop that appears at the anodes. It is well known that this voltage drop results from the negative space charge that builds up around the anode and is equivalent to the added energy that must be given to the electrons to counteract the repelling force of the space charge. It is also well known that the magnitude of the space charge and resultant anode voltage drop increase as the size of the anode decreases. Hence, the voltage drop can be minimized by using an enlarged anode mounted near the cathode.

In order to avoid the added cost, the loss of light output and other disadvantages associated with the use of such enlarged anodes, it has been proposed that an elec trically floating grid be mounted in front of the cathode to relieve the space charge and thus reduce the voltage drop when the cathode serves as the anode on alternate half cycles of the operating A.C. voltage. A fluorescent lamp incorporating such a grid structure and the manner in which the latter modifies the character of the discharge in the vicinity of the anodes is disclosed in US. Patent No. 2,725,497, entitled Floating Grids for Fluorescent Lamps, and issued to J. J. Mason on Nov. 29, 1955. The desired reduction in the anode voltage drop is achieved in accordance with the teachings of this patent by mounting a wire mesh grid in front of the cathode, or alternatively, enclosing the latter in a so-called cage grid.

While the use of such single-grid structures has given satisfactory results, it would be desirable to reduce the anode drop to an even greater extent and minimize the resultant power loss. The problem has been aggravated by the recent development of fluorescent lamps having mixed fill gases instead of the argon fill customarily e. ployed. Such lamps contain a fill gas comprising a mixture of argon and a lighter gas such as neon or helium. The addition of the lighter gas increases the wattage in the positive column portion of the discharge and thus increases the light output. However, such mixed fill gases also increase the electrode losses and, when used in lamps having conventional type cathodes without auxiliary anodes, results in a total lamp wattage that exceeds the design limitations of existing ballasts designed for use with conventional argon-filled lamps.

In view of the foregoing, it is the general object of the present invention to provide an improved gaseous discharge lamp that wi-ll avoid the aforementioned disadvantages of the prior art lamps.

Another object is the provision of a fluorescent lamp having cathodes of conventional design and space charge relieving means that reduces the anode voltage drop to a value such that the lamp can be filled with a mixed starting gas and operated on existing ballasts designed for use with argon-filled lamps without exceeding the wattage rating of such ballasts.

Still another object is the provision of an improved grid assembly that can be conveniently and inexpensively fabricated and subsequently mounted on the stem of an electric discharge lamp to relieve the space charge in front of the associated lamp electrode.

The foregoing objects, and other advantages that will become apparent to those skilled in the art as the description proceeds, are achieved in accordance with the present invention by mounting a pair of grids foraminous members in spaced tandem relationship a predetermined distance in front of each of the fluorescent lamp cathodes. In a preferred embodiment these members comprise grids that are made from wire mesh and are spaced a predetermined distance apart so as to define a region within the discharge which counteracts the repelling force of the negative space charge adjacent the cathodes. The tandemspaced grids thus extend the space charge relieving effect over a region of the discharge equivalent to the volume included between the grids and, accordingly, reduce the anode voltage drop by a factor much greater than that achieved by a single grid. The total lamp wattage is thus maintained Within the desired limits without the use of separate anodes.

A better understanding of the invention will be obtained by referring to the accompanying drawing, Where in:

FIGURE 1 is an elevational view of a fluorescent lamp which incorporates the present invention, portions of the envelope being omitted for convenience of illustration;

FIG. 2 is an enlarged perspective view of one of the electrode mounts used in the lamp shown in FIG. 1; and

FIG. 3 is a perspective view of an alternative grid assembly according to the invention.

With specific reference to the drawing, in FIG. 1 there is shown a fluorescent lamp 10 comprising a tubular lighttransmitting envelope 12 having a coating 14 of suitable ultraviolet-responsive phosphor on its inner surface and an electrode mount 16 sealed into each of its ends. Each of these mounts include the usual pair of lead-in wires 18 and 19 that are connected to and support the cathode 20 and are sealed through the glass stem 17. The outer ends of the leads are connected to a pair of pins 22 carried by a base 24 attached to the sealed ends of the envelope 12. The envelope is filled with a suitable starting gas, such as argon or a mixture thereof with neon or helium, and is also charged with a predetermined quantity of mercury in accordance with standard lamp-making procedures.

The cathodes 20 are preferably elongated coils of tungsten wire that extend perpendicular to the lamp axis and are coated with a suitable emission material, such as a mixture of alkaline earth oxides. The cathodes are of the type conventionally employed in argon-filled lamps and, since no auxiliary anodes are used, operate as anodes on alternate half cycles of the operating A.C. voltage.

Embodiment I In accordance with the present invention, the anode voltage drop is markedly reduced by a pair of foraminous members such as a pair of substantially planar wire mesh grids 26 and 27 that are mounted in tandem relationship a predetermined distance d in front of the respective cathodes 20, as shown in FIG. 1. As illustrated, the wire mesh grids 26 and 27 are held in substantially aligned relationship by a rigid support wire 28 that is fastened to the re spective mounts 16. The grids are thus electrically connected to each other and are disposed in parallel relationship with the associated cathodes and extend perpendicularly across the discharge path between the cathodes. The aforesaid grids are preferably of circular configuration and slightly smaller than the diameter of the envelope 12 so that they extend across the entire cross-section of the envelope and the discharge.

As shown more particularly in FIG. 2, the paired grids 26 and 27 are preferably held in the aforesaid position by a pair of rigid support wires 28 and 29 that are embedded in opposite sides of the press 21 formed on the inner end of the glass stem 17 and extend therefrom on opposite sides of the cathode 20 to the peripheral edges of the grids to which they are fastened, as by welding. The grids are thus electrically isolated from both cathodes and may be termed floating grids.

The grids 26 and 27 are preferably woven from nickelfree stainless steel wire approximately mils in diameter. The wire dimensions and tightness of the mesh are preferably such that the smallest dimension of the openings in the grids is at least A of an inch. However, grids having openings /8 inch and even Mr inch on a side can be used.

The spacing between the grids (dimension x in FIG. 1) is critical and should be maintained between about 0.06 and 0.25 inch, depending on the thickness of the wire used for the grid. The spacing between the cathodes and the closest one of the pair grids (dimension d in FIG. 1) is also critical and should be maintained between about 0.1 to 0.5 inch if the grids are to operate with maximum effectiveness.

Embodiment II In FIG. 3 there is shown an alternative form of grid assembly 30 which comprises a pair of generally circular hoops 31 and 32 of stiff wire or the like held in parallel spaced and aligned relationship by a plurality of wire studs 34 that are fastened as by welding to the peripheral edges of the hoops. A plurality of fine wires 36 and 38 are fastened to the respective hoops and form a pair of spaced grid structures that are located in the planes of the respective hoops. The hoops, and thus the grids, are spaced a predetermined distance apart, as indicated by the dimension x in FIG. 3.

Comparative tests conducted on fluorescent lamps having a norminal rating of 40 watts and conventional cathodes with no auxiliary anodes and a fill gas consisting of 40% neon-60% argon at 2.5 mm. pressure have shown that the use of a tandem-grid versus a single grid reduces by the wattage dissipated at the ends of the lamp due to electrode losses. A single grid located A" in front of the cathodes reduced the total lamp wattage by 2 watts, whereas a pair of mesh grids spaced A apart and mounted A in front of the respective cathodes produced a 3 watt reduction.

4 It will be apparent from the foregoing that the objects of the invention have been achieved in that an inexpensive and simple grid assembly has been provided which reduces the electrode losses in a fluorescent lamp to a value heretofore unattainable in the prior art. The lamp efi'iciency is thus increased and the wattage of mixed fill gas lamps can readily be maintained within the rating limits of existing ballasts designed for conventional argon filled lamps.

While several embodiments have been illustrated and described, it will be understood that various changes in both the construction and arrangement of the grid assemblies can be made without departing from the spirit and scope of the invention. For example, while wire mesh grids have been referred to in the foregoing disclosure, electrically conductive grids are not necessary so that the grids can be fabricated from fibers of insulating material such as glass or the like. Also, instead of a plurality of planar wire meshes, a spiral mesh or a perforated strip with /8 square openings can be used.

I claim:

1. A low-pressure electric discharge lamp comprising:

a light-transmitting envelope containing an ionizable medium,

a pair of spaced electrodes sealed within said envelope one of which functions as an anode during the operation of the lamp, and

a pair of foraminous members supported in spaced tandem relationship with respect to one another adjacent said one electrode which functions as an anode,

said foraminous members being electrically isolated from both of said electrodes and oriented so that they extend transversely across the discharge path between said electrodes,

said foraminous members having a spacing therebetween of from about 0.06 inch to 0.25 inch and being spaced from said one electrode a distance such that the portion of the discharge space located between said members constitutes a region in the discharge that relieves the negative space charge adjacent said one electrode when the latter is functioning as an anode and reduces the anode voltage drop.

2. The electric discharge lamp set forth in claim 1 wherein said fora'minous members are fabricated from electrically conductive material and are connected to one another. I

3. The electric discharge lamp set forth in claim 1 wherein; said envelope is of tubular configuration, and said foraminous members extend across substantially the entire cross-section of said envelope.

4. In a fluorescent lamp adapted for operation on an alternating-current power source, the combination comprising:

an electrode that alternately operates as a cathode and an anode during the operation of the lamp, and

a pair of spaced electrically-floating grids of substantially planar configuration supported in the discharge space predetermined distances in front of said electrode,

said grids being disposed in tandem relationship across the discharge path and the spacing therebetween being in the range of about 0.06 to 0.25 inch whereby said grids define a region along the discharge path that reduces the negative space charge adjacent said electrode when the latter operates as an anode.

5. The combination set forth in claim 4 wherein the spacing between said electrode and the closest of said grids is within the range of from about 0.1 to 0.5 inch.

6. The electric discharge lamp set forth in claim 1 wherein; said lamp comprises a fluorescent lamp, said ionizable medium includes a fill gas comprising a mixture of argon and neon or a mixture of argon and helium, and the reduction in the anode voltage drop decreases the total lamp wattage by a predetermined 2,725,497 11/1955 Mason 313-204 amount. 2,966,602 12/1960 Waymouth et a1. 313226 X References Cited 3,101,426 8/1963 Takayama 313207 X UNITED STATES PATENTS FOREIGN A E T 1,607,276 11/1926 Hewitt 313-204 744,387 2/1956 Great Britain. 2,136,292 11/1938 Gabor 313-204 2,185,189 1/ 1940 Germeshausen 313204 JOHN W. HUCKERT, Primary Examiner. 2,363,070 11/1944 Lemmers 313204 2,530,990 11/1950 Peters 313 204 JAMES,ASS1Sl"fExammer- 

